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Welding of Magnesium-base Alloys

Abstract:

Magnesium can be welded by many of the arc and resistance welding processes, as well as by the oxyfuel gas welding process, and it can be brazed.Magnesium like aluminum is produced with different tempers. These are based on heat treatment and work hardening. The strength of a weld joint is lowered in base metal, in the work-hardened condition, as a result of recrystalization and grain growth in the heat-affected zone. This effect is minimized with gas metal arc welding because of the higher welding speed utilized. This is not a factor in the base metals that are welded in the soft condition.

Magnesium alloys containing small amounts of aluminum, manganese, zinc,
zirconium, etc., have strength equaling that of mild steels. They can be
rolled into plate, shapes, and strip. Magnesium can be cast, forged,
fabricated, and machined.

As a structural metal it is used in aircraft. It is used by the
materials-moving industry for parts of machinery and for hand-power tools
due to its strength to weight ratio. Magnesium can be welded by many of
the arc and resistance welding processes, as well as by the oxyfuel gas
welding process, and it can be brazed.

Magnesium like aluminum is produced with different tempers. These are
based on heat treatment and work hardening. The strength of a weld joint
is lowered in base metal, in the work-hardened condition, as a result of
recrystalization and grain growth in the heat-affected zone. This effect
is minimized with gas metal arc welding because of the higher welding
speed utilized. This is not a factor in the base metals that are welded
in the soft condition.

Magnesium possesses properties that make welding it different than
the welding of steels. Many of these are the same as for
aluminum. These are:

Magnesium oxide surface coating.

High thermal conductivity.

Relatively high thermal expansion coefficient.

Relatively low melting temperature.

The absence of color change as temperature approaches the melting point.

The normal metallurgical factors that apply to other metals apply to magnesium as well.

Magnesium is a very active metal and the rate of oxidation increases as
the temperature is increased. The melting point of magnesium is very
close to that of aluminum, but the melting point of the oxide is very
high. In view of this, the oxide coating must be removed.

Magnesium has high thermal heat conductivity and a high coefficient
of thermal expansion. The thermal conductivity is not as high as
aluminum but the coefficient of thermal expansion is very nearly
the same. The absence of color change is not too important with
respect to the arc welding processes.

The welds produced between similar alloys will develop the full strength
of the base metals, however, the strength of the heat-affected zone may
be reduced slightly. In all magnesium alloys the solidification range
increases and the melting point and the thermal expansion decrease as
the alloy content increases.

In the magnesium-aluminum-zinc alloys (AZ31B, AZ61A, AZ63A, AZ80A, AZ81A,
AZ91 and AZ92A), aluminum content up to about 10% aids weldability by
helping to refine the grain structure, while zinc content of more than
1% increases hot shortness, which may cause weld cracking.

The high zinc alloys (ZH62A, ZK51A, ZK60A and ZK61A) are not recommended
for arc welding because they are highly susceptible to cracking and have
poorer weldability. Magnesium, containing small amounts of thorium,
possesses excellent welding qualities and freedom from cracking.
Weldments of these alloys do not require stress relieving.

Certain magnesium alloys are subject to stress corrosion. Weldments subjected to
corrosive attack over a period of time may crack adjacent to welds if
the residual stresses are not removed. For weldments intended for this
type of service stress relieving is required.

The gas tungsten arc welding process and the gas metal arc welding process
are the two recommended processes for joining magnesium. Gas tungsten arc
is recommended for thinner materials and gas metal arc is recommended for
thicker materials, however, there is considerable overlap. The equipment
for applying these processes has been previously described.

Filler Metals

The four most commonly used electrode wires for gas metal arc welding (GMAW)
and filler metals (when used) for gas tungsten arc welding (GTAW) are
ER AZ61A, ER AZ101A, ER AZ92A and ER EZ33A. The choice of electrode
wire or filler metal is governed by the composition of the base metal.

Electrode wires or filler metals having composition conforming to
ER AZ61A or ER AZ92A (Mg-Al-Zn) are considered satisfactory for welding
alloys AZ10A, AZ31B, AZ31C, AZCOML, AZ61A, AZ80A, ZE10A and ZK21A to
themselves or to each other. ER AZ61A is usually preferred for welding
aluminum-containing wrought products because of tendency to resist crack
sensitivity. The ER AZ92A filler metal shows less crack sensitivity for
welding the cast magnesium-aluminum-zinc and magnesium-aluminum alloys.

The same electrode wires or filler metals are used for joining any one
of the above alloys to high-temperature alloys HK31A, HM21A, and HM31A.
However, when the high temperature alloys are joined to each other, ER
EZ33A is recommended. Joints of wrought or cast alloys welded with ER
EZ33A filler metal exhibit good mechanical properties at high temperatures.

The choice of electrode wire or filler metal for welding wrought alloys
to cast alloys should be based on the recommendations outlined above,
except that ER AZ101A may be used instead of ER AZ61A or ER AZ92A.

When aluminum-containing cast alloys are joined to aluminum-containing
cast alloys, ER AZ101A or ER AZ92A electrode wire or filler metal is
usually recommended. However, for joining HK31A and HZ32A to themselves
or to each other, ER EZ33A is preferred, for joining HK31A and HZ32A to
any of the other cast alloys, ER AZ101A is used. Filler rod of the same
composition as the base metal should be used for most welds.

Gas Tungsten Arc Welding

All the precautions mentioned for welding aluminum should be observed.
A short arc should be used and the torch should have a slight leading
travel angle. The cold wire filler metal should be brought in as near
to horizontal as possible (on flat work). The filler wire is added to
the leading edge of the weld puddle.

High frequency current should be used for starting the direct current and
arc with alternating current high frequency should be used continuously.
Runoff tabs are recommended for welding any except the thinner materials.
Uniform travel speed and weld beads are recommended.

The shielding gas is normally argon. However, a mixture of 75% helium
plus 25% argon is used for thicker materials. For heavy thicknesses
100% helium can be used, more helium is required than argon to do the
same job.

Gas Metal Arc Welding of Magnesium

The gas metal arc welding process is used for the medium to thicker
sections. It is considerably faster than gas tungsten arc welding. Special
high-speed gear ratios are usually required in the wire feeders since
the magnesium electrode wire has an extremely high meltoff rate. The
normal wire feeder and power supply used for aluminum welding will be
suitable for welding magnesium.

The different types of arc transfer can be obtained when welding magnesium.
This is primarily a matter of current level or current density and voltage
setting. The short-circuiting transfer and the spray transfer should be
used on material 3/16 inch and thicker and the short-circuiting arc used
for thinner metals.

Other Welding Processes

The resistance welding processes can be used for welding magnesium,
including spot welding, seam welding, and flash welding. Magnesium can
also be joined by brazing. Most of the different brazing techniques can
be used. In all cases, brazing flux is required and the flux residue must
be completely removed from the finished part. Soldering is not too
popular since the strength of the joint is relatively low.

Magnesium can be stud welded, gas welded, and plasma welded. Finely
divided pieces of magnesium such as shavings, fillings, etc, should
not be in the welding area since they will burn. Magnesium castings,
or wrought materials do not create a safety hazard since the possibility
of fire caused by welding on these sections is very remote. The producers
of magnesium provide additional data for welding magnesium.